Measuring extracellular resistance in a chamber of defined volume containing cells has been used to monitor physiological conditions. In a method identified by O'Connor et al. (hereinafter O'Connor) and subsequently applied by others, adherent cells on a solid substrate were placed in a hand-made chamber. The resistance of the extracellular fluid in the chamber was then measured using an AC current with phase detection. Swelling of the cells reduced the cross sectional area of the extracellular liquid in the chamber. Taken over the length of the cell, this was observed as an increase in the measured resistance of the chamber.
While the resistance method identified by O'Connor is capable of non-invasively sampling a large adherent cell population and generally provides real time recording, the sensitivity of O'Connor's method and device, however, is limited.